The lithographic printing is a method of alternately supplying a fountain solution and an oily ink to the surface of a lithographic printing plate having a surface consisting of a lipophilic image part and a hydrophilic non-image part, the hydrophilic non-image part working as a fountain solution-receiving part (ink non-receiving part) and the ink being received only in the lipophilic image part by utilizing the repellency between water and oil from each other, and then transferring the ink to a material on which an image is printed, such as paper, thereby performing printing.
For producing this lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon a lipophilic photosensitive resin layer (image recording layer) has been heretofore widely used. Usually, a lithographic printing plate is obtained by a plate-making method where the lithographic printing plate precursor is exposed through an original image such as lith film and while leaving the image recording layer in the portion working out to the image part the other unnecessary image recording layer is dissolved and removed with a developer such as alkaline aqueous solution or organic solvent to expose the hydrophilic support surface and thereby form the non-image part.
In the plate-making process using a conventional lithographic printing plate precursor, a step of dissolving and removing the unnecessary image recording layer with a developer or the like must be provided after exposure and as one problem to be solved, it is demanded to dispense with or simplify such an additive wet processing. In particular, the treatment of waste solutions discharged along with the wet processing is recently a great concern to the entire industry in view of consideration for global environment and the demand for solving the above-described problem is becoming stronger.
As one of simple plate-making methods to cope with such a requirement, a method called on-press development has been proposed, where an image recording layer allowing for removal of the image recording layer of a lithographic printing plate precursor in a normal printing process is used and after exposure, the unnecessary image recording layer is removed on a printing press to obtain a lithographic printing plate.
Specific examples of the on-press development method include a method using a lithographic printing plate precursor having an image recording layer dissolvable or dispersible in a fountain solution, an ink solvent or an emulsified product of fountain solution and ink, a method of mechanically removing the image recording layer by the contact with rollers or a blanket cylinder of a printing press, and a method of weakening the cohesion of the image recording layer or adhesion between the image recording layer and the support by the impregnation of a fountain solution, an ink solvent or the like and then mechanically removing the image recording layer by the contact with rollers or a blanket cylinder.
In the present invention, unless otherwise indicated, the “development processing step” indicates a step where, by using an apparatus (usually an automatic developing machine) except for a printing press, the unnecessary portion of the image recording layer is removed through contact with a liquid (usually an alkaline developer) to expose the hydrophilic support surface, and the “on-press development” indicates a method or step where, by using a printing press, the unnecessary portion of the image recording layer is removed through contact with a liquid (usually a printing ink and/or a fountain solution) to expose the hydrophilic support surface.
On the other hand, a digitization technique of electronically processing, storing and outputting image information by using a computer has been recently widespread and various new image-output systems coping with such a digitization technique have been put into practical use. Along with this, a computer-to-plate (CTP) technique is attracting attention, where digitized image information is carried on a highly converging radiant ray such as laser light and a lithographic printing plate precursor is scan-exposed by this light to directly produce a lithographic printing plate without intervention of a lith film. Accordingly, one of important technical problems to be solved is to obtain a lithographic printing plate precursor suitable for such a technique.
As described above, the demand for a simplified, dry-system and non-processing plate-making work is ever-stronger in recent years from both aspects of consideration for global environment and adaptation for digitization.
To satisfy this requirement, for example, Japanese Patent No. 2,938,397 describes a lithographic printing plate precursor where an image forming layer comprising a hydrophilic binder having dispersed therein hydrophobic thermoplastic polymer particles is provided on a hydrophilic support. In Japanese Patent No. 2,938,397, it is stated that after exposing this lithographic printing plate precursor with an infrared laser to cause coalescence of hydrophobic thermoplastic polymer particles by the effect of heat and thereby form an image and then loading it on a cylinder of a printing press, the lithographic printing plate precursor can be on-press developed with use of a fountain solution and/or an ink. However, in such a method of forming an image through coalescence by mere heat fusion of fine particles, despite good on-press developability, the image strength is low and the press life is not satisfied.
For solving these problems, a technique of improving the press life by utilizing a polymerization reaction has been proposed. For example, JP-A-2001-277740 describes a lithographic printing plate precursor comprising a hydrophilic support having thereon an image recording layer (thermosensitive layer) containing a polymerizable compound-enclosing microcapsule, and JP-A-2002-29162 describes a lithographic printing plate precursor comprising a support having provided thereon an image recording layer (photosensitive layer) containing an infrared absorbent, a radical polymerization initiator and a polymerizable compound.